Gas turbine fuel injector with insulating air shroud

ABSTRACT

A fuel injector for a gas turbine engine is disclosed. The fuel injector includes an injector housing extending from a first end to a second end along a longitudinal axis. The second end of the housing is fluidly coupled to a combustor of the turbine engine and the housing includes a liquid fuel gallery annularly disposed about the longitudinal axis. The fuel injector also includes a stem extending longitudinally from the first end of the housing to a third end. The stem includes a liquid tube configured to deliver liquid fuel to the fuel injector. The fuel injector also includes an annular shell extending along the longitudinal axis from the first end to the third end and circumferentially disposed about the stem. The fuel injector further includes an insulating air shroud formed inside the shell. The air shroud includes a layer of air between the shell and the stem.

TECHNICAL FIELD

The present disclosure relates generally to a fuel injector for a gasturbine engine, and more particularly, to a gas turbine fuel injectorwith an insulating air shroud.

BACKGROUND

Gas turbine engines produce power by extracting energy from a flow ofhot gas produced by combustion of fuel in a stream of compressed air. Ingeneral, turbine engines have an upstream air compressor coupled to adownstream turbine with a combustion chamber (“combustor”) in between.Energy is released when a mixture of compressed air and fuel is ignitedin the combustor. The resulting hot gases are directed over blades ofthe turbine, spinning the turbine, thereby, producing mechanical power.In typical turbine engines, one or more fuel injectors direct some typeof liquid or gaseous hydrocarbon fuel (such, diesel fuel or natural gas)into the combustor for combustion. Some embodiments of fuel injectorsare designed to direct both a liquid and a gaseous fuel into thecombustor. In these embodiments, the turbine engine may operate on onefuel as the primary fuel with the other fuel used during periods ofunavailability of the primary fuel. For example, some gas turbineengines may normally operate on natural gas fuel. In these turbineengines, diesel fuel may be used during periods of natural gasunavailability. The fuel is mixed with compressed air (from the aircompressor), in the fuel injector, and delivered to the combustor forcombustion. This compressed air, which may exceed 800° F. (426.7° C.) intemperature, may surround sections of the fuel injector, and may createa hot ambient environment for the fuel injector. Combustion of the fuelin the combustor creates hot gases exceeding 2000° F. (1093.3° C.),which may heat surrounding surfaces. The heat released due to combustionmay also heat fuel injectors, which may be coupled to the combustor.

Fuel injectors include fuel lines and fuel galleries that are used todirect the fuel to the fuel injector and deliver the fuel to thecombustor. In a fuel injector that is configured to deliver both liquidand gaseous fuel to combustor, separate fuel lines may deliver theliquid and gaseous fuel to the fuel injector. When the turbine engineoperates on gaseous fuel, the liquid fuel may remain in the fuel linesand galleries. In some embodiments, the liquid fuel may be purged fromthe liquid fuel lines and galleries. However, even in these embodiments,the liquid fuel may exist as a coating on these purged lines andgalleries. Due to operating conditions of the fuel injector, the liquidfuel in the liquid fuel lines and galleries may be exposed to ambienttemperatures of about 500° F.-800° F. (260° C.-426.7° C.) and injectorsurface temperatures of 1000° F.-2000° F. (537.8° C.-1093.3° C.). Thishigh temperature may lead to coking of the liquid fuel in the lines andgalleries. Over time, the coke may deposit on the lines and galleriesand lead to flow restrictions and inoperable conditions.

U.S. Pat. No. 7,117,675 ('675 patent), a patent issued to Kaplan et al.on Oct. 10, 2006, describes a cooling system for gas turbine liquid fuelcomponents to prevent coking. In the system of the '675 patent, a sleevesurrounds a liquid fuel component and a device is used to provide acurrent of cool air through a space between the liquid fuel componentand the sleeve. In the cooling system of the '675 patent the sleevesurrounding the liquid fuel component includes a plurality of spacersfor centering the sleeve around the liquid fuel component to create anannulus between the sleeve and the liquid fuel component, through whichthe current of cool air flows. The current of cool air that is used tocool the liquid fuel component is directed to the annular space using aconduit connected between the cool air device and the sleeve. Althoughthe cooling system of the '675 patent may prevent coking of the liquidfuel within the liquid fuel component, it may have some drawbacks. Forinstance, using a cool air device to blow cool air around the liquidfuel component may increase the complexity and cost of operating theturbine engine. In addition using individual sleeves to provide anannular space around each liquid fuel component may introduce designcomplexities when space is limited.

SUMMARY

In one aspect, a fuel injector for a gas turbine engine is disclosed.The fuel injector includes an injector housing extending from a firstend to a second end along a longitudinal axis. The second end of thehousing is fluidly coupled to a combustor of the turbine engine and thehousing includes a liquid fuel gallery annularly disposed about thelongitudinal axis. The fuel injector also includes a stem extendinglongitudinally from the first end of the housing to a third end. Thestem includes a liquid tube configured to deliver liquid fuel to thefuel injector. The fuel injector also includes an annular shellextending along the longitudinal axis from the first end to the thirdend and circumferentially disposed about the stem. The fuel injectorfurther includes an insulating air shroud formed inside the shell. Theair shroud includes a layer of air between the shell and the stem.

In another aspect, a method of operating a gas turbine engine isdisclosed. The method includes delivering liquid fuel to a combustor ofthe turbine engine through one or more liquid fuel carrying componentsof a fuel injector coupled to the combustor, and combusting the liquidfuel in the combustor. The method also includes providing an insulatingair shroud around one or more of the liquid fuel carrying components,and generating eddy air currents in the insulating air shroud inresponse to the combustion. The eddy air currents expel heated air fromthe insulating air shroud and draw cooler air into the insulating airshroud. The method further includes maintaining a temperature of the oneor more liquid fuel carrying components below a threshold temperature asa result of the generation of the eddy air currents.

In yet another aspect, a method of assembling a fuel injector to a gasturbine engine is disclosed. The method includes fluidly coupling asecond end of an injector housing to a combustor of the turbine engine.The housing extends from a first end to the second end along alongitudinal axis and the housing includes a stem that extendslongitudinally from the first end to a third end. The stem includes aliquid tube configured to deliver liquid fuel to the fuel injector. Themethod also includes coupling an annular shell to the housing at thefirst end. The shell extends along the longitudinal axis from the firstend to the third end and is circumferentially disposed about the stem toform an insulating air shroud inside the shell. The air shroud includesa layer of air between the shell and the stem. The method furtherincludes coupling the annular shell to an outer casing of the turbineengine at the third end to form a compressed air space in an areaoutside the shell. The shell prevents flow of air between the compressedair space and the air shroud.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an exemplary disclosed gas turbine enginesystem;

FIG. 2 is a cross-sectional view of a fuel injector of the turbineengine of FIG. 1;

FIGS. 3A and 3B illustrate cross-sectional views of the first end andsecond end respectively of the fuel injector of FIG. 2; and

FIG. 4 is an a cross-sectional view of a embodiment of a shell of thefuel injector of FIG. 2.

DETAILED DESCRIPTION

FIG. 1 illustrates an cut away view of an exemplary gas turbine engine(turbine engine) 100. Turbine engine 100 may have, among other systems,a compressor system 10, a combustor system 20, a turbine system 70, andan exhaust system 90. In general, compressor system 10 may compressincoming air to a high pressure, combustor system 20 may mix thecompressed air with a fuel and burn the mixture to producehigh-pressure, high-velocity gas, and turbine system 70 may extractenergy from the high-pressure, high-velocity gas flowing from thecombustor system 20.

Compressor system 10 may include any device capable of compressing air.In some embodiments this may include an axial flow compressor thatproduces a continuous flow of compressed air. The axial flow compressormay include rotating and stationary components that cooperate tocompress air to the required pressure. A central shaft 12, disposedconcentrically about a longitudinal axis 88, may drive a central drum 14of compressor system 10. The central drum 14 may have a number ofannular aerofoils 16 attached thereon in rows along longitudinal axis88. These aerofoils 16 may rotate between similar rows of stationaryaerofoils 16 attached to a stationary tubular casing of compressorsystem 10. Typically, the rotating aerofoils 16 are called “rotors” andthe stationary aerofoils 16 are called “stators.” Atmospheric air mayenter compressor system 10, and pass through these aerofoils 16. As theair flows through aerofoils 16, the air may get compressed and airpressure may increase. Along with increased pressure, the compressed airexiting aerofoils 16 may have a high temperature. The high pressure andhigh temperature air may exit compressor system 10 through an outletport 18. A pair of rotating and stationary aerofoils is called a stage.In general, the pressure and temperature of air exiting outlet port 18may depend, among others, on the number of stages of compressor system10. In some embodiments, the pressure and temperature of air exitingcompressor system 10 may exceed 200 psi and 800° F. (426.7° C.)respectively.

Combustor system 20 may be connected to outlet port 18 of compressorsystem 10. Combustor system 20 may include an annular combustor 50disposed about longitudinal axis 88. In some embodiments, combustorsystem 20 may include multiple substantially cylindrical combustors(called can-type combustors) arranged in a circular array pattern aboutlongitudinal axis 88. In some embodiments, combustor system 20 mayinclude combustors that are a hybrid of annular and can-type combustors(combination type combustor). Although an annular combustor 50 isdepicted in FIG. 1, the disclosed fuel injector with an insulatingshroud may be applicable with any type of combustor. Outlet port 18 ofcompressor system 10 may deliver compressed air into an enclosure 22formed by an outer casing 24 around central shaft 12. Compressed airfrom enclosure 22 may be directed into one or more fuel injectors 30coupled to combustor 50 and annularly positioned about longitudinal axis88.

FIG. 2 illustrates a cross-sectional view of a fuel injector 30 coupledto combustor 50. Fuel injector 30 may be positioned in enclosure 22 witha first end 45 coupled to combustor 50 and a second end 35 coupled toouter casing 24. High pressure and high temperature compressed air fromcompressor system 10 may surround fuel injector 30 in enclosure 22. Insome cases, the temperature of compressed air in enclosure 22 may exceed800° F. (426.7° C.). This high temperature compressed air may heatexternal surfaces of fuel injector 30.

The compressed air in enclosure 22 may be directed into fuel injector 30through an air swirler 42. Air swirler 42 may include a plurality ofstraight or curved blades attached to a housing 30 a of fuel injector 30to swirl the incoming compressed air. The number of blades in airswirler 42 may vary with application. Although air swirler 42 of FIG. 2is illustrated as a radial swirler, air swirler 42 in general, may havea radial or an axial configuration. A radial swirler is an air swirlerin which compressed air from compressor system 10 may be directed to thecurved blades radially, while an axial swirler is an air swirler inwhich the compressed air may be directed to the curved blades axially.

A plurality of liquid fuel nozzles 58 attached to housing 30 a mayinject liquid fuel into the swirled air stream from air swirler 42.Although liquid fuel nozzles 58 positioned upstream of air swirler 42are depicted in FIG. 2, in some embodiments, these liquid fuel nozzles58 may take the form of small tubes attached to air swirler 42. Fuelinjector 30 may also include gas ports (not shown) to deliver thegaseous fuel to combustor 50. In some embodiments, these gas ports mayinclude a plurality of small holes located on air swirler 42. Whenturbine engine 100 operates using gaseous fuel, fuel gas may be injectedinto the swirled air stream through these gas ports. Swirling theincoming air into fuel injector 30, using air swirler 42, may help mixthe fuel with the compressed air and deliver a premixed mixture of fueland air to combustor 50. This premixed fuel-air mixture may be deliveredto combustor 50 through a premix barrel 32 of fuel injector 30 that maybe coupled to combustor 50.

Fuel injector 30 may also include a pilot assembly 40 disposed radiallyinwards of premix barrel 32. In some embodiments, pilot assembly 40 andpremix barrel 32 may be aligned along a second longitudinal axis 98 offuel injector 30. Pilot assembly 40 may include components configured toinject a stream of pressurized fuel into combustor 50. In embodiments offuel injector 30 configured to deliver both liquid and gaseous fuel tocombustor 50, pilot assembly 40 may be configured to inject a stream ofpressurized liquid and gaseous fuel into combustor 50. Pilot assembly 40may also include components configured to deliver a stream of compressedair along with the pressurized fuel into combustor 50. In addition,swirl features (not shown) may also be located within pilot assembly 40to swirl the compressed air delivered to pilot assembly 40.

Combustor 50 may include an ignition device (not shown), such as a torchigniter, to ignite the fuel delivered to combustor 50. The premixedfuel-air mixture delivered through premix barrel 32, and the pressurizedstream of fuel and air delivered through pilot assembly 40, may ignitein combustor 50 to create combustion flames. Once ignited, a continuousstream of fuel delivered through fuel injector 30 may sustain thecombustion flame. An average temperature of the combustion flame may, insome cases, exceed 2000° F. (1093.3° C.). The flame may heat surfaces ofcombustor 50 and first end 45 of fuel injector 30 proximate the flame.This heat may be transferred to relatively cooler regions of the fuelinjector 30 by standard modes of heat transfer (such as, conduction,convection, and radiation). A cooling air flow may be maintained througha space between multiple walls (not shown) of combustor 50 to keep thecombustor surfaces at a safe operating temperature.

Fuel injector 30 may include fuel supply conduits that deliver fuel tofuel injector 30. These conduits may form a stem 34 extendinglongitudinally from second end 35 along second longitudinal axis 98. Thestem 34 may include a main gas tube 48, a pilot gas tube, main liquidfuel tube 54, and pilot liquid tube 44. It is contemplated that, in someembodiments, stem 34 may include less than, or more than, these aforementioned conduits. In some embodiments, stem 34 may extend along secondlongitudinal axis 98 from second end 35 towards housing 30 a. Main gastube 48 may supply gaseous fuel from a gaseous fuel manifold (not shown)to a main gas gallery 52 included in fuel injector housing 30 a. Maingas gallery 52, annularly positioned around second longitudinal axis 98,may deliver gaseous fuel to the swirled air stream in premix barrel 32.Main gas gallery 52 may also supply gaseous fuel to pilot assembly 40.In some embodiments, a separate pilot gas tube included in stem 34 maysupply gaseous fuel to pilot assembly 40.

Liquid fuel tube 54 may supply liquid fuel from a liquid fuel supply(not shown) to a main liquid gallery 56 included in housing 30 a. Mainliquid gallery 56 may include an annular channel around secondlongitudinal axis 98. Main liquid gallery 56 may be fluidly coupled toliquid fuel nozzle 58 and may deliver liquid fuel to the swirled airstream in premix barrel 32 to create the premixed fuel-air mixture.

Pilot liquid tube 44 may direct liquid fuel from outside fuel injector30 to pilot assembly 40. Pilot liquid tube 44 may be an elongateassembly extending from second end 35 to first end 45 along secondlongitudinal axis 98. The liquid fuel delivered to pilot assembly 40through pilot liquid tube 44 may be sprayed into combustor 50 through anozzle coupled to first end 45 of pilot liquid tube 44. Compressed airmay also be injected into combustor 50 alongside the fuel spray throughopenings around pilot liquid tube 44. This liquid fuel and compressedair spray may form the pressurized stream of fuel and air delivered tocombustor 50 through pilot assembly 40.

Heat transferred from the combustion flame (in combustor 50) and thecompressed air (in enclosure 22) to the relatively cooler regions offuel injector 30 may heat the liquid fuel carrying components of fuelinjector 30. The term “liquid fuel carrying components” are generallyused to include any component of fuel injector 30 that is configured todeliver liquid fuel to combustor 50. In some embodiments, these liquidfuel carrying components may include liquid fuel tube 54, main liquidgallery 56, liquid fuel nozzle 58, and pilot liquid tube 44. It iscontemplated that, in some embodiments, liquid fuel carrying componentsmay include additional liquid fuel carrying components, or less than allthe afore mentioned liquid fuel carrying components. It may be desirableto keep the temperature of some (or all) of these liquid fuel carryingcomponents below a threshold temperature during operation of the turbineengine 100. In general, this threshold temperature may be any value oftemperature. In some embodiments, the threshold temperature may be about400° F. (204.4° C.). Maintaining a temperature of the liquid fuelcarrying components below about 400° F. (204.4° C.) may prevent cokingof the liquid fuel in the liquid fuel carrying components.

A shell 72 may be coupled to fuel injector 30 to form an insulating airshroud 74 around the liquid fuel carrying components to keep theirtemperature below about 400° F. (204.4° C.). Shell 72 may extendlongitudinally from second end 35 of fuel injector 30 to a third end 65,proximate air swirler 42. Shell 72 may be coupled to housing 30 a atthird end 65 and to a circular disk 62 at second end 35. In someembodiments, shell 72 may be brazed to housing 30 a at third end 35.However, other methods of coupling shell 72 to housing 30 a are alsocontemplated. FIGS. 3A and 3B illustrate sections of fuel injector 30 atthird end 65 and second end 35, respectively. In the description thatfollows, reference will be made to both FIGS. 3A and 3B. Circular disk62 may be coupled to stem 34 and may include passageways to pass stem 34there-through. Air gaps 76 (shown in FIG. 3B) may be formed between stem34 and circular disk 62. These air gaps 76 may vent insulating airshroud 74 to atmosphere outside outer casing 24.

Insulating air shroud 74 may include a space formed between shell 72 andstem 34 of fuel injector 30. Insulating air shroud 74 may include alayer of air that shields the liquid fuel carrying components from thetemperature of the combustor 50 and the temperature of the compressedair in enclosure 22. The air in insulating air shroud 74 may get heatedby the heat transferred from combustor 50 and enclosure 22. The heatedair proximate third end 65 may interact with cooler air towards secondend 35. The interaction of heated air with the cooler air may createnatural eddy currents within the space. These eddy currents may allowthe heated air in the space to escape through air gap 76. These eddycurrents may also draw in cooler air atmospheric air (from theatmosphere outside outer casing 24) into insulating air shroud 74through air gap 76. The eddy currents may keep air in insulating airshroud 74 relatively cool, and maintain the temperature of the liquidfuel carrying components below about 400° F. (204.4° C.).

FIG. 4 illustrates a cross-sectional view of an exemplary shell 72 usedin an application. Shell 72 may be made of any material that willsurvive the temperatures and stresses induced during operation ofturbine engine 100. In some embodiments, shell 72 may be made of astainless steel alloy, such as, for example 316L stainless steel alloy.Shell 72 may enclose substantially all the liquid fuel carryingcomponents within insulating air shroud 74. Although the size and shapeof shell 72 may depend upon the application, in some embodiments, shell72 may have a length 82 between about 9 to 10 inches (22.9 to 25.4centimeters). Shell 72 may have a generally tubular shape with a firstdiameter 84 at the second end 35, and a second diameter 86 at third end65, respectively. At a location between the second end 35 and third end65, shell 72 may have a third diameter 92 less than first diameter 84and second diameter 86. Although in general, these diameters may dependupon the application, in some embodiments shell 72 may have firstdiameter 84 between about 3.5 to 4.5 inches (8.9 to 11.4 centimeters),second diameter between about 4 to 5 inches (10.2 to 12.7 centimeters),and third diameter between about 1.5 to about 2.5 inches (3.8 to 6.4centimeters). The resulting shape of shell 72 may provide an insulatingair shroud 74 where eddy currents may be established to keep atemperature of the liquid fuel carrying components below about 400° F.(204.4° C.) while reducing the overall size of shell 72.

Shell 72 may include a flange section 78 at second end 35 of fuelinjector 30. The flange section 78 may extend substantiallyperpendicularly away from second longitudinal axis 98. In someembodiments, flange section 78 may include fastener holes 78A annularlyin a circular array about second longitudinal axis 98. The flangesection 78 may be used to couple fuel injector 30 to outer casing 24 ofturbine engine 100 (shown in FIG. 2). In some embodiments, fasteners(not shown) passing through fastener holes 78A in flange section 78 maybe used to attach fuel injector 30 to outer casing 24. Structural loadsfrom fuel injector 30 may be transferred to outer casing 24 primarilythrough shell 72. Although, in the exemplary embodiments describedherein, insulating air shroud 74 is configured to maintain a temperatureof the liquid fuel carrying components below 400° F. (204.4° C.), ingeneral, an insulating air shroud of the current disclosure may beconfigured to maintain a temperature of any component of a turbineengine fuel injector below any threshold temperature.

INDUSTRIAL APPLICABILITY

The disclosed gas turbine fuel injector with an insulating air shroudmay be applicable to any turbine engine where it is desirable tomaintain a temperature of selected regions of the fuel injector below adesired temperature. In an embodiment of a fuel injector that isconfigured to deliver liquid fuel to the turbine engine, an insulatingair shroud may be used to maintain the temperature of all or selectedliquid fuel carrying components below about 400° F. (204.4° C.), andthereby prevent coking of the liquid fuel. The operation of a gasturbine engine with a fuel injector having liquid fuel carryingcomponents maintained below about 400° F. (204.4° C.) will now bedescribed.

During operation of turbine engine 100, air may be drawn into turbineengine 100 and compressed in compressor system 10 (see FIG. 1).Compression of the air may increase a temperature of the compressed airto about 800° F. (426.7° C.). The compressed air may be directed to anenclosure 22 of the turbine engine 100. The hot compressed air inenclosure 22 may heat a fuel injector 30 located in enclosure 22. Thecompressed air from enclosure 22 may be directed to a combustor 50 ofcombustor system 20 through fuel injector 30. Fuel may be mixed with thecompressed air as it flows through fuel injector 30 into combustor 50.The fuel-air mixture may burn in combustor 50 producing a temperature ofabout 2250° F. (1232.2° C.).

A shell 72 may be coupled with fuel injector 30 to shield the liquidfuel carrying components (liquid fuel tube 54, main liquid gallery 56,liquid fuel nozzle 58, and pilot liquid tube 44 of FIG. 2) of fuelinjector 30 from the heat of combustion and the hot compressed air inenclosure 22. Shell 72 may couple with housing 30 a of fuel injector 30to form an insulating air shroud 74 around the liquid fuel carryingcomponents. The air in the insulating air shroud 74, proximate third end65, may get heated by the combustion of the fuel-air mixture incombustor. This heated air in the insulating air shroud may interactwith cooler air near the second end 35 and set up eddy currents withinthe insulating air shroud 74. These eddy air currents may expel hot airfrom the insulating air shroud 74 and draw cooler air into theinsulating shroud 74 to maintain the temperature of the liquid fuelcomponents below about 400° F. (204.4° C.).

Creating an insulating air shroud around liquid fuel carrying componentsof the fuel injector enables the temperature of these components to bemaintained below 400° F. (204.4° C.), and thereby prevent coking.Although temperatures of regions in close proximity to the liquid fuelcarrying components may be at a significantly higher temperature, theinsulating air shroud keeps the liquid fuel components relatively cool.Since cooling of the liquid fuel carrying components occurs due to anatural phenomenon of air within the insulating air shroud (that is,without the aid of external air moving means), the cost associated withpreventing coke formation in liquid fuel components of the turbineengine may be low. Additionally, the shell that creates the insulatingair shroud may be designed to meet the space requirements of fuelinjectors 30.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the disclosed fuel injectorwith insulating air shroud. Other embodiments will be apparent to thoseskilled in the art from consideration of the specification and practiceof the disclosed fuel injector with insulating air shroud. It isintended that the specification and examples be considered as exemplaryonly, with a true scope being indicated by the following claims andtheir equivalents.

1-9. (canceled)
 10. A method of operating a gas turbine engine,comprising: delivering liquid fuel to a combustor of the turbine enginethrough one or more liquid fuel carrying components of a fuel injectorcoupled to the combustor; combusting the liquid fuel in the combustor;providing an insulating air shroud around one or more of the liquid fuelcarrying components, the insulating air shroud including a layer ofatmospheric air; generating eddy air currents in the insulating airshroud in response to the combustion, the eddy air currents assisting inexpelling heated atmospheric air from the insulating air shroud anddrawing cooler atmospheric air into the insulating air shroud; andmaintaining a temperature of the one or more liquid fuel carryingcomponents below a threshold temperature as a result of the generationof the eddy air currents.
 11. The method of claim 10, wherein providingan insulating air shroud includes providing an insulating air shroudbetween the one or more liquid fuel carrying components and a shell. 12.The method of claim 11, wherein the delivering of liquid fuel includesdelivering a liquid fuel to the combustor through a liquid fuel tubecoupled to the fuel injector.
 13. The method of claim 12, wherein theexpelling of heated air includes expelling heated air from theinsulating air shroud through one or more air gaps formed between theinsulating shell and the liquid fuel tube, and the drawing of cooler airincludes drawing cooler air into the insulating air shroud through theair gaps.
 14. The method of claim 10, wherein providing an insulatingair shroud includes providing an insulating air shroud between one ormore of the liquid fuel carrying components and an enclosure containingcompressed air.
 15. The method of claim 10, wherein the maintaining of atemperature includes maintaining a temperature of at least one of theliquid fuel carrying components below about 400° F. (204.4° C.).
 16. Themethod of claim 10, wherein the delivering of liquid fuel to a combustorincludes; delivering liquid fuel to a liquid fuel gallery of the fuelinjector through a liquid tube; and delivering liquid fuel to a pilotassembly of the fuel injector through a pilot liquid tube.
 17. Themethod of claim 16, wherein the forming of an insulating air shroudincludes forming an insulating air shroud over one or more of the liquidfuel gallery, the liquid tube, and the pilot liquid tube. 18-20.(canceled)
 21. A method of operating a gas turbine engine, comprising:directing liquid fuel and compressed air to a combustor of the gasturbine engine through a fuel injector, the fuel injector extending froma first end coupled to the combustor to a third end along a longitudinalaxis; directing the liquid fuel to the third end of the fuel injectorthrough a stem, the stem extending along the longitudinal axis from thethird end, in a direction away from the first end, to a second end suchthat the third end is located between the second end and the first end,the stem including a liquid tube configured to pass the liquid fueltherethrough; and providing an insulating air shroud circumferentiallyabout the stem, the insulating air shroud being formed by a layer ofatmospheric air in an annular space between the stem and an annularshell circumferentially disposed about the stem.
 22. The method of claim21, wherein providing the insulating air shroud includes closing theannular space at the third end to prevent flow of the atmospheric airinto the combustor.
 23. The method of claim 22, wherein providing theinsulating air shroud further includes providing one or more openings atthe second end to vent the atmospheric air in the annular space to theatmosphere.
 24. The method of claim 21, further including providing acovering member at the second end to at least partially cover theannular space between the shell and the stem at the second end.
 25. Themethod of claim 21, wherein providing an insulating air shroud includesshielding the shell from an enclosure of the gas turbine enginecontaining compressed air using the layer of atmospheric air.
 26. Themethod of claim 21, wherein directing the liquid fuel and compressed airincludes directing compressed air from the enclosure into the combustorthrough the fuel injector.
 27. The method of claim 21, further includinggenerating eddy air currents in the insulating air shroud in response tocombustion in the combustor, the eddy air currents assisting inexpelling heated atmospheric air from the insulating air shroud anddrawing cooler atmospheric air into the insulating air shroud.
 28. Amethod of operating a gas turbine engine, comprising: delivering liquidfuel to a combustor of the turbine engine through one or more liquidfuel carrying components of a fuel injector; delivering compressed airto the combustor through the fuel injector; combusting a mixture of theliquid fuel and the compressed air in the combustor; and providing aninsulating air shroud around the one or more liquid fuel carryingcomponents of the fuel injector, the insulating air shroud including alayer of atmospheric air that separates the compressed air directed tothe combustor from the one or more liquid fuel carrying components. 29.The method of claim 28, further including generating eddy air currentsin the insulating air shroud in response to the combustion in thecombustor, the eddy air currents assisting in expelling heatedatmospheric air from the insulating air shroud and drawing cooleratmospheric air into the insulating air shroud.
 30. The method of claim28, wherein providing an insulating air shroud includes maintaining atemperature of the one or more liquid fuel carrying components below acoking temperature of the liquid fuel.
 31. The method of claim 28,wherein providing an insulating air shroud includes preventing the flowof the atmospheric air in the insulating air shroud into the combustor.32. The method of claim 31, wherein providing an insulating air shroudfurther includes providing one or more openings to fluidly couple thelayer of atmospheric air in the insulating air shroud to the atmosphere.